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equant
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Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background.
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equant

astronomy
Also known as: equant point
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Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background.
Ptolemaic system In Ptolemy's geocentric model of the universe, the Sun, the Moon, and each planet orbit a stationary Earth. For the Greeks, heavenly bodies must move in the most perfect possible fashion—hence, in perfect circles. In order to retain such motion and still explain the erratic apparent paths of the bodies, Ptolemy shifted the centre of each body's orbit (deferent) from Earth—accounting for the body's apogee and perigee—and added a second orbital motion (epicycle) to explain retrograde motion. The equant is the point from which each body sweeps out equal angles along the deferent in equal times. The centre of the deferent is midway between the equant and Earth.

The term "equant" in astronomy refers to a point used in the Ptolemaic system, a geocentric model of the universe developed by Ptolemy around 150 CE. This system assumed Earth was stationary at the center of the universe. To explain the observed irregular movements of planets, Ptolemy's model combined circular motions. The equant was a key artifact in this model, allowing the center of a planet's epicycle (a small circle on which the planet moves) to sweep out equal angles in equal times as seen from the equant point. This point was located midway between the center of the deferent (the larger circle around Earth) and Earth itself. While the equant helped accurately predict planetary motion, it was controversial and later challenged by astronomers like Copernicus and Kepler, whose work eventually led to the heliocentric model and elliptical orbits.1

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6 Britannica Sources
  1. equant
  2. Ptolemaic system
  3. universe - Astronomical theories of the ancient Greeks
  4. celestial mechanics - Newton’s laws of motion
  5. Equuleus
  6. astronomy - Prehistory

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